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Biological clock. Conceptual image depicting human biorhythms, showing a clock superimposed on a woman's naked body. Many biological processes are believed to take place under the direction of an internal clock mechanism. Processes such as wakefulness and tiredness are based on a daily or circadian (24 hour) cycle regulated by hormones such as melatonin and cortisol. Another example is the menstrual cycle, which repeats itself roughly every 28 days in fertile women, also controlled by hormones.